The present invention relates to a method of reducing the stress of a thin film such as a tungsten thin film formed in a semiconductor manufacturing process and, more particularly, is concerned with a method of reducing the stress of a blanket tungsten thin film formed by chemical vapor deposition (CVD) process.
In recent years, a tungsten film forming technique based on CVD process has widely been employed in manufacturing of semiconductors, since it is excellent in step coverage, rich in resistance to migration, and easy to etch. For example, the tungsten film (hereinafter referred to as xe2x80x9cW filmxe2x80x9d) formed by CVD process has been used for establishing interlayer connection of multilayer wiring by embedding a contact hole or through hole with tungsten.
Conventionally, a method of forming blanket W by CVD comprises the steps of forming a nucleus from tungsten hexafluoride (WF6), monosilane (SiH4), and hydrogen (H2); and subsequently growing blanket W by use of WF6 and H2. As a condition for nucleation, the flow ratio of WF6/SiH4 is required to be at least 1 in order to prevent vapor phase reactions from occurring.
In general, a film located immediately under the W film is an insulating film. It has been becoming more often that the insulating film is formed by use of organic materials or inorganic materials which are easily degassed. Therefore, while the forming of blanket W by CVD has conventionally been carried out at about 450xc2x0 C., processing at a lower temperature of about 400xc2x0 C. is in demand since degassing occurs from the substrate itself or insulating film, and so forth. However, there is a problem that the tensile film stress becomes higher when the film is formed at a lower temperature of about 400xc2x0 C.
If the thin film formed in a semiconductor manufacturing process has a stress higher than a reference value, which fluctuates between the steps before and after the target step, then microcracks may occur in transistor sections, wiring sections, and the like due to warpage of the silicon wafer, thereby creating inconveniences in focusing at an exposure step subsequent thereto.
In view of the foregoing circumstances, it is an object of the present invention to provide a method of controlling the stress of a blanket W film, by which a film characteristic on a par with the film forming at a higher temperature is obtained without enhancing the film stress even when the film forming processing is carried out at a lower temperature.
The inventors have carried out various studies in order to achieve this object and, as a result, have inferred that, if processing for forming a film is effected at a lower temperature, then a volume contraction occurs due to the merging of gain boundaries, which raises the stress of the film.
Accordingly, the present invention provides a method of forming a blanket W film on a substrate by CVD process, the method including the steps of: evacuating a vacuum chamber containing a substrate to a predetermined degree of vacuum; carrying out preprocessing of the substrate by introducing a first mixed gas containing WF6 gas and SiH4 gas at a predetermined ratio with a WF6 gas/SiH4 gas flow ratio adjusted to 1 or less into the vacuum chamber for a predetermined period of time; forming a nucleus of a blanket W film on the substrate by introducing a second mixed gas containing WF6 gas and SiH4 gas at a predetermined ratio into the vacuum chamber; and then growing a blanket W film on the substrate formed with the nucleus by introducing a third mixed gas containing WF6 gas and H2 gas at a predetermined ratio.
In accordance with the present invention, before the nucleation of the blanket W film, the preprocessing is carried out for a predetermined period of time with the first mixed gas containing WF6 gas and SiH4 gas. It is assumed that, due to the preprocessing, the nucleus of tungsten to be formed later becomes greater, so as to reduce the area of crystal boundaries, thereby lowering the stress of the grown blanket W film. Therefore, even when the processing for forming the blanket W film is carried out at a lower temperature, the film stress would not increase, whereby a film stress on a par with that in the processing at a higher temperature can be maintained.
For obtaining such a stress reducing effect, the WF6 gas/SiH4 gas flow ratio in the first mixed gas is preferably within the range of 0.15 to 0.35, more preferably within the range of 0.2 to 0.3.
The preprocessing using the first mixed gas is carried out preferably for 2 to 10 seconds, more preferably for 3 to 8 seconds.
The first mixed gas may further contain H2 gas, whereas the second mixed gas may contain H2 gas as well.
The term xe2x80x9csubstratexe2x80x9d is applied in this specification not only to substrates themselves but also to those in which other films such as insulating films are formed on substrates.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading the following detailed description when taken in conjunction with the attached drawings.